U.S. Pat. No. 4,034,475 discloses a system wherein jaw movements are recorded on a small sheet or plate overlying a patient's temporo-mandibular joint, and that information is then used to position preformed guide blocks in a dental articulator. Such blocks have average value guide openings for receiving the styluses of the articulator which define or represent a hinge axis.
In one approach the recorded curve of protrusive path is traced onto a transparent sheet which is then mounted on the upper frame of a dental articulator so that the patient's hinge axis is aligned with the axis defined by the styluses of the lower frame of a dental articulator. A reference line on the recorded plate which was in the patient's horizontal reference plane when the protrusive path was recorded is aligned with a corresponding reference plane on the dental articulator. The analogue guide blocks in the upper frame of the articulator which receive the styluses of the lower frame each have a curved upper wall which determines the protrusive path and operation of the articulator. The curve, which has a radius close to that of a significant percentage of the population, is marked on the exterior of the guide block in some suitable fashion. To complete the setting of the guide block in the articulator, the guide block is rotated until the curve on its exterior surface is aligned with the curve representing the patient's protrusive path which is marked on the tracing that is positioned adjacent the curve on the guide block. Once the curves are aligned, as best as they can be, the guide block is fixed in that position, followed by setting of the guide block on the other side of the articulator. The operation of the articulator is then used to simulate the patient's jaw movements.
While the above-described system has many advantages over systems that had previously been used for making jaw movement measurements and for setting dental articulators, the several steps take time and are sources for error. For example, the simplest way of obtaining the tracing of the patient's protrusive curve on the transparent sheet is to position the transparency over the record plate and manually trace the curve onto the transparency. While this is direct, it is dependent to some extent on the skill and care of the operator. Secondly, means must be provided on the dental articulator for mounting the transparency on the articulator in proper position with respect to the articulator horizontal plane of reference. The above-mentioned patent discloses a special fixture for this purpose which is temporarily mounted on the upper frame of the articulator and then removed after the guide blocks have been properly set. This of course introduces some equipment expense and adds to the time required to set the articulator. There is also the expense of marking or indicating on the exterior of the guide block the curvature of the upper wall of the guide block.
Gysi, as recorded in the January 1910 issue of the Dental Cosmos discloses a dental articulator wherein a pair of guide plates each having curved paths formed therein are used to guide a horizontal shaft which is part of the upper frame of a dental articulator. The curved slots are said to be average value representations of movements of a patient's lower jaw. Paths of a patient's jaw movements are obtained on a record card, and by trial and error, the guide plate having a curved slot most closely conforming to that of the recorded path is physically aligned over the recorded path. Two pointers on the plate define a line which is approximately parallel to a central portion of the curved slot. Points at tips of the pointers are marked on the recording sheet; and after the guide plate is removed from the sheet, a line is drawn between the two points and extended to intersect the lower edge of the recording sheet, to form an angle with respect to the lower edge. The lower edge was positioned parallel to the plane of occlusion of the patient's jaws when the paths of mandibular movement were recorded on the sheet. Thus, the angle formed on the card is an angle of a portion of a mandibular movement curve with respect to the plane of occlusion. A protractor is then used to measure the angle, and this measurement is used to set the angle of the guide plates when they are mounted on a dental articulator.
Although there are some advantages to the Gysi system, there are also some shortcomings. The line defined by the pointers on the Gysi guide plate is somewhat arbitrarily positioned in that it varies as the curve varies. Further, the system requires marking a measurement with the protractor after the angle has been formed; and the angle obtained is with respect to the plane of occlusion rather than with respect to a plane through the patient's hinge axis. More importantly, the Gysi articulator is not of the type currently being used.